1. Field of the Invention
The present invention relates generally to processes for producing and recovering organic acids, such as lactic acid. More particularly, it concerns methods that rely on azeotropic distillation for production and recovery of organic acids from a feed stream comprising an organic acid amide, an organic acid ammonium salt, or an alkyl amine-organic acid complex.
2. Description of Related Art
Organic acids such as lactic acid have a number of commercial uses, for example in food manufacturing, pharmaceuticals, plastics, textiles, and as starting materials in various chemical processes. The current market in the United States for a particular organic acid, lactic acid, is about 50,000 tons per year, more than half of which is imported. It is well known to produce organic acids by fermentation of sugars, starch, or cheese whey, using microorganisms such as Lactobacillus delbrueckii to convert monosaccharides such as glucose, fructose, or galactose, or disaccharides such as sucrose, maltose, or lactose, into organic acids such as lactic acid. The broth that results from fermentation contains unfermented sugars, carbohydrates, amino acids, proteins, and salts, as well as the acid. Some of these materials cause an undesirable color or can interfere with downstream processing of the organic acid. The acid usually therefore must be recovered from the fermentation broth and in some cases must undergo further purification before it can be used.
Lactic acid and other α-hydroxyacids exist in two different optical isomers. For the example of lactic acid, these isomers are L-(+)-lactic acid and D-(−)-lactic acid. An equal mixture of D and L lactic acids is called a racemic mixture. It is often desirable to produce lactic acid with a high proportion of only one of the optical isomers. Different microorganisms used in fermentations to produce organic acids can produce different proportions of optical isomers of a particular organic acid. Chemical synthesis to prepare a higher proportion of a particular optical isomer can be difficult. It is desirable to minimize reactions that lead to the conversion of L-(+)-lactic acid into D-(−)-lactic acid and vice versa, so called racemization reactions. Exposing lactic acid solutions to relatively high temperatures can increase certain racemization reactions.
During the production of an organic acid such as lactic acid by fermentation, the increasing concentration of the acid in the fermentation broth reduces the pH. As the pH decreases, the growth of the microorganism is inhibited and eventually stops, and therefore acid production stops. To prevent this, the pH of the fermentation broth typically is controlled by adding a base for neutralization, such as ammonia or a sodium or calcium base. However, one result of the addition of such a base is the formation of a salt of the acid (e.g., ammonium lactate). Therefore, it is often necessary to convert the salt to free acid or another form such as an ester, which subsequently can be converted to the free acid.
It is known in the art that alkylamine can be used to aid in the recovery of organic acids from fermentation broths or other streams comprising organic acids and their salts (e.g., hydrolyzed polylactide, among others) via extraction. These amines are thought to interact with organic acid in an organic phase (e.g., extract) to form what can be termed an alkylamine-organic acid complex. It is known in the art that both ion pair and hydrogen bond interactions occur between the organic acid and the amine in the alkylamine-organic acid complex. In certain cases, the solvent phase of such an extraction can, in addition to the alkylamine, also comprise a diluent such as an alkane or an aromatic species. The solvent phase of such an extraction can also optionally comprise an extraction enhancer such as 1-octanol, which can enhance the solubility of the alkylamine-organic acid complex in the solvent phase.
As noted above, the alkylamine can be used to aid in removing acids from an aqueous phase (e.g. a fermentation broth). Therefore, methods to produce the organic acid from the alkylamine-organic acid complex of such an extraction are desirable.
Lactic acid is one organic acid of particular interest today because of a great projected demand for use as a polymer feedstock, particularly for use in producing degradable plastics. It is also used in the pharmaceutical and food industries, in leather tanning and textile dyeing, and in making solvents, inks, and lacquers. Although lactic acid can be prepared by chemical synthesis, production of lactic acid by fermentation of starch, cane sugar, whey or certain other carbon sources is a less expensive method. The production of lactic acid by fermentation is most efficient at a pH range where the lactic acid is largely present as a salt. Thus recovery of pure lactic acid often requires conversion of the salt into free acid and additional purification steps. One method that is used in purification is the production of a lactate ester from the lactic acid or salt, followed by purification of the ester. Finally the ester is converted to the free acid.
Hydroxyacids, such as lactic acid, or diacids can be converted to polymers (e.g., polyesters). These polymers can be recycled via digestion using pressurized water, acid, base, or a combination of such treatments. The product of such digestion can be a mixture of organic acids, salts of organic acids, and amides of organic acids. This digested material, which can be recycled for use in other processes, can contain significant impurities and can require purification to recover the organic acids therein.
Additionally, during processing of ammonium salts of organic acids, there is a tendency for organic amides to form, for example lactamide is formed from lactic acid via the following reaction:Lactic acid+ammonia→lactamide+water
There is a long standing need for improved processes for the production and recovery of relatively pure organic acids, particularly lactic acid, from feed streams comprising at least one of an organic acid amide, a alkyl amine-organic acid complex, and/or an ammonium salt of an organic acid.